There have conventionally been apparatuses that irradiate focused laser light on a sample, image scattered light from the sample, perform predetermined image processing on the picked up image, and measure the density distribution of defective particles in the sample based on the image processing result (refer to Patent Document 1). According to this method, as shown in FIG. 3 or FIG. 4, a two-dimensional image is obtained using the 90-degree scattering method by which it is possible to know the two-dimensional placement of defective particles by imaging the laser light that is scattered by the defective particles in the sample.
Patent Document 1: JP 2604607 (Japanese unexamined patent application, First Publication, No. 01-151243)
Patent Document 2: JP 2832269 (Japanese unexamined patent application, First Publication, No. 06-094595)
With a defective particle of a large size, the effective scattering cross-sectional area that scatters the incident laser light is large, and therefore the scattering intensity is large. For this reason, in a defective particle scattering image in which the scattering intensity is large, the defective particle can generally be determined to be one having a large size.
However, laser light that is incident on the sample has a light intensity distribution on a plane that is perpendicular to the axis of incidence of the laser light. Therefore, even for defective particles of the same size, the scattering intensity from a defective particle that is located away from the axis of incidence decreases compared to the scattering intensity from a defective particle that is in the vicinity of the axis of incidence. For this reason, even if only the scattering intensity of the defective particle scattering images is measured, the size of the defective particle cannot be determined. That is, the defective particle scattering image and the defective particle size do not directly correspond.
For example, FIG. 18 shows a scattering intensity distribution when the beam diameter of the incident laser light is 8 μm. Defective particle SB lies on the axis of incidence, while defective particle SA is located 8 μm away from the axis of incidence. The defective particle SA is large and has a scattering efficiency that is 100 times that of the defective particle SB. Curve LA is the scattering intensity distribution of the defective particle SA, and curve LB is the scattering intensity distribution of the defective particle SB. In this case, even though the sizes of the defective particles SA and SB differ by 100 times, the measured scattering intensities are approximately 7, and thus the same scattering intensity. Therefore, it is not possible to determine the size of the defective particles only by the scattering intensity.
For this reason, in the three-dimensional particle detection method disclosed in Patent Document 2, the sample is shifted in the depth direction and a plurality of the defective particle scattering images obtained in Patent Document 1 are acquired as cross-sectional images. By subjecting these cross-sectional images to three-dimensional image processing, the nonuniformity in the optical intensity distribution that is input is corrected, and the size of each defective particle is determined.
However, since it is necessary to obtain a plurality of cross-sectional images in the method that is disclosed in Patent Document 2, the problem arises of the measurement taking time.